The heart of a computer is a magnetic disk drive which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
In typical systems, recession is created after lapping and pre-carbon etching processes. This creates a distance between the transducers and the disk surface. Thermal fly-height control (TFC) is a method of altering this distance between the transducers and the disk surface by heating the components of the reader/writer causing thermal expansion of the materials, which results in the reader/writer transducers protruding closer to the surface of the hard disk. The transducers are moved closer to the disk surface to enable proper reading and writing of the tracks.
To accurately control the head to disk clearance, the power to the TFC heater may be calibrated until head-disk contact is initiated, noting the necessary heating power for contact, and then reducing the power to achieve the desired clearance. Power is supplied to the heater until a portion of the head protrudes and makes contact with the disk. This contact establishes a zero of the spacing. The heat is then reduced until the read sensor retracts from the disk by a pre-determined amount. This spacing change at the reader location can be measured by the change in the readback signal using the Wallace spacing law, as is known to those of ordinary skill in the relevant art. This TFC protrusion calibration may require a series of head-disk contacts either during the manufacture of the disk drive or during disk drive operations. Unfortunately, this conventional method of TFC, which is now commonly used to control the slider disk spacing in head disk drives, does not provide the absolute spacing of the reader from the disk, as the contact location can be away from the reader. This flying height difference between the point of contact and the reader location is a source of uncertainty and should be minimized without making the reader itself the point of contact.
In prior art systems, during TFC induced touchdown, there is a higher probability of contact between the disk and the AlTiC edge or the reader. The AlTiC is much harder than the disk, and therefore damage to the disk overcoat or the disk itself may result from contact and create a risk of disk corrosion or defective sites that may cause a crash or loss of data. Also, contact between the reader or writer pole-tip and the hard disk can damage the reader/writer and cause performance issues due to magnetostriction or may cause the device to malfunction or operate erratically. Similarly, contact with the reader/writer may cause the carbon overcoat to wear off, possibly leading to corrosion of the head. Therefore, a method of determining when the pole-tip is in proper position without risking damage to useful portions of the hard disk drive or magnetic head is desired.